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Identifier 000402871
Title Development of graphene-based materials and their application in organic solar cells
Alternative Title Ανάπτυξη γραφενικών δομών και η εφαρμογή τους σε οργανικά φωτοβολταϊκά
Author Κωνιός, Δημήτριος Κ.
Thesis advisor Κουτσολέλος, Αθανάσιος
Abstract Ever since the isolation of free standing graphene in 2004, graphene research has experienced a phenomenal growth. Its exceptional electronic, optical and mechanical properties make graphene highly attractive, believed to be the next wonder material and thus triggering the application of graphenebased materials in the different layers of optoelectronics and especially organic photovoltaics (OPVs). In this thesis, novel graphene derivatives have been developed towards all graphene-based photovoltaics. First of all, aiming to improve the processability of graphene oxide (GO) and reduced graphene oxide (rGO), the solubility of GO and rGO in a large number of common organic solvents was investigated. Their dispersions were prepared and compared, with respect to the long-term stability and dispersion quality. The effect of reduction process on the solubility of GO was investigated considering the solvent polarity and the surface tension. This work contribution mainly lies in the fact that for the first time, the solubility values of both GO and rGO were calculated and the data was analyzed to identify the Hansen and Hildebrand solubility parameters for the two graphene derivatives, facilitating the application of graphene derivatives to printed flexible electronics. To contribute to the existing research on the use of graphene as transparent conductive electrode in OPVs, a novel, one step laser-based method to pattern previously prepared rGO thin films was presented. In more detail, the micromesh (MM) patterning of the rGO films with fs UV laser pulses, resulted in a significant increase of the transparency, retaining at the same time their conductivity at high levels, thereby improving the tradeoff between rGO layers transparency and sheet resistance. In particular, rGO films with initial transparency of ~20% were patterned, resulting in rGOMMs films with ~59% transmittance and sheet resistance of ~565 Ωsq−1, significantly lower than the pristine rGO films resistance (~780 Ωsq−1), exhibited at the same transparency. As a proof-of-concept application, rGOMMs were used as the transparent electrodes in flexible OPV devices, achieving power conversion efficiency (PCE) of 3.05%, the highest ever reported for flexible OPV devices incorporating solution-processed graphene-based electrodes. The interface between the active layer and the electrodes plays an important role in the overall device performance of organic electronics. In this context, the effective utilization of work function (WF) tuned solution processable graphenebased derivatives as buffer layer in OPV devices was also demonstrated. The systematic tuning of functionalized GO WF was performed by either photochlorination for WF increase, or lithium neutralization for WF decrease. In this way, the WF of the photochlorinated GO (GO-Cl) layer perfectly matched with the HOMO level of two different polymer donors, enabling excellent hole transport. On top of that, the WF of the lithium functionalized GO (GO-Li) perfectly matched with the LUMO level of the fullerene acceptor, enabling excellent electron transport. The utilization of these graphene-based hole and electron transport layers in OPV devices, led to significant PCE improvement (+19.5% compared to PEDOT:PSS HTL, +14.2% compared to devices without the GO-Li interfacial layer, +19% in combo devices with GO-Cl HTL and GO-Li interfacial ETL). Finally, the synthesis of graphene-inorganic nanocrystal derivatives as a way of designing energetically favorable materials for solar cells applications was also demonstrated. In particular, the synthesis and the application of reduced graphene oxide-antimony sulfide (rGO-Sb2S3) hybrid nanosheets as the cascade material in ternary PCDTBT:PC71BM-based OPV led to PCE of 6.81%. The rGO-Sb2S3 hybrids combine the advantages of the individual materials, and could potentially enhance the electron cascade transfer into the active layer
Language English
Subject Additive
Buffer layers
Graphene derivatives
Organic photovoltaic
Transparent conductive electrode
Work function
Έργο εξόδου
Διάφανο αγώγιμο ηλεκτρόδιο
Ενδιάμεσα στρώματα
Οργανικές φωτοβολταϊκές διατάξεις
Παράγωγα γραφενίου
Issue date 2016-10-13
Collection   Faculty/Department--Faculty of Sciences and Engineering--Department of Chemistry--Doctoral theses
  Type of Work--Doctoral theses
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